Costco stores start earlier than almost any retailer in carrying Halloween and Christmas items. I think the main reason is that, being a warehouse store, Costco has nowhere to store inventory. So what do they do with the holiday inventory once it comes in? Put it on the floor for early-bird shoppers to snag. I don’t think Costco minds this too much, because they can start their holiday sales early, which can be better for numbers.
So far for Halloween I have seen a large decorative skull (pictured), some decorative lawn stakes, and some kid’s costumes. I think the costumes are already sold out at my local Costco. Meanwhile for Christmas items, I have seen ribbon for wrapping gifts for a while now. We just got in glass ornaments, oversized outdoor ornaments, LED multi-color lights, a Santa sculpture, and nutcracker tealights. I highly recommend if you see anything you like, buy it right away. I cannot stress how quickly seasonal items sell out at Costco. If you’re debating on it, buy it, and if you don’t like it or don’t end up using it, you can return it later. One other tip is that you can look at the item tag/price tag, and if it has a star on it that means it’s discontinued and won’t be restocked. I hope you find what you’re looking for.